1. Field of the Invention
The present invention relates to a method of producing a cellulose acylate dispersion used for preparing a swelling solution and a dope, more especially to a method of producing a cellulose acylate dispersion used for preparing a swelling solution and a dope for forming a film for optical use in-a solution casting method.
2. Description Related to the Prior Art
Cellulose acylate, especially cellulose triacetate (hereinafter TAC) is used as a raw material for producing a TAC film in a solution casting method. The TAC film is used as a base of a photosensitive material and in a liquid crystal display. In a method of producing the TAC film, the TAC is dissolved to a mixture solvent whose main solvent is dichloromethane, to prepare a polymer solution (hereinafter dope). Then the dope is cast on a support (for example a belt, a rotary drum, and the like) to form a casting film. When the casting film has a self-supporting property, the casting film is peeled from the support by a peel roller, dried and cooled. Thereafter the casting film is wound as a film (see, Japan Institute of Invention and Innovation (JIII) JOURNAL No.2001-1745).
The dope is prepared in a batch operation in which TAC powders and the solvent are usually supplied into a tank and then stirred and mixed. In the batch operation, there are some merits, for example, the quality of the solution is uniform in one batch, and the amount of the preparation in each batch is observed such that the concentration and composition of the solution are regulated. However, in the stirring in the tank, the undissolved aggregates (lumps) generated in the aggregation of the powders are easily generated. Otherwise in order to reduce the generation of the undissolved aggregates, the TAC may be added at the small amount each several times to the solvent. In this case, the TAC which is supplied later is fed into the solution in which the TAC is dissolved. Thus the penetration and swelling of the solvent into the TAC is more hardly proceeded in the solution than the solvent in which the TAC is not dissolved. Accordingly, in this case, the solubility of the TAC becomes lower in the progress of the dissolution. Therefore the TAC and the solvent are continuously supplied into a first vessel, and the mixing and stirring is made to swell for one or more minute. Thereafter the obtained mixture is fed into a second vessel and heated. Thus the TAC is entirely dissolved in the solvent, to make the obtained solution uniform. (see, Japanese Patent Laid-Open Publication No.2000-273239.
However, in the method described in the publication No.2000-273239, when the retention time in the first vessel is one minute or more, it is hard to prevent the generation of the undissolved aggregates. Further, when the undissolved aggregates are generated once, the undissolved aggregates as the aggregated materials are in a stable situation. In this case, even when the heating is made in the second vessel, the dissolution of the undissolved aggregates is hard. Further, in the batch operation, while the stirring is made in the tank, the mixture cannot be fed into the next process which is continuously driven. Therefore the buffer tank is necessary. However, the quantity in the buffer tank is in and decreased repeatedly, and therefore the solution remaining on an inner wall of the tank is dried to generate skinnings, gel-like materials and the like.